This proposal is a request for funds to purchase a Becton Dickinson FACStar Plus Research Flow Cytometer equipped with dual lasers (argon ion laser and dye laser), an automatic cell deposition unit and microVAX II GPX two user station with associated hardware and software. This versatile and powerful fluorescence-activated cell sorter system will provide the means for a large and highly productive and innovative users group to more efficiently pursue and achieve the objectives of their presently funded research programs. These programs range from the regulation of hematopoiesis, including the development of megakaryocytes, macrophages and lymphocytes, and the regulation of immunoglobulin synthesis, to the immunobiology of lymphoid and myeloid cells. both in animal models and in children with leukemia. Many of these programs are at critical stages where the availability of the requested instrument will allow for both the efficient achievement of the immediate research objectives as well as the development of new and novel directions in the future programs.